Coupled N and P dynamics in a forested headwater stream.
Scientists at ORNL examined the effects of single and dual nitrogen and phosphorus additions on nutrient cycling in a co-limited (i.e., for N and P) headwater stream (Walker Branch, TN).
There is a growing need to investigate coupled biogeochemical cycles, especially in ecosystems that may be co-limited (e.g., for N and P). This novel research approach used two nutrient addition techniques to investigate coupled N and P cycling in stream reaches and may be applied to other elemental cycles and environmental settings.
Nitrogen (N) and phosphorus (P) can limit autotrophic and heterotrophic metabolism in lotic ecosystems, yet most studies that evaluate biotic responses to colimitation focus on patch-scale (e.g., nutrient diffusing substrata) rather than stream-scale responses. In this study, ORNL scientists evaluated the effects of single and dual N and P additions on ambient nutrient uptake rates and saturation kinetics during two biologically contrasting seasons (spring, autumn) in Walker Branch, a temperate forested headwater stream in Tennessee, USA. In each season, they used separate instantaneous pulse additions to quantify nutrient uptake rates and saturation kinetics of N (nitrate) and P (phosphate). The team then used steady-state injections to elevate background stream water concentrations (to low and then high background concentrations) of one nutrient (e.g., N) and released instantaneous pulses of the other nutrient (e.g., P). We predicted that elevating the background concentration of one nutrient would result in a lower ambient uptake length and a higher maximum areal uptake rate of the other nutrient in this co-limited stream. Their prediction held true in spring, as maximum areal uptake rate of N increased with elevated P concentrations from 185 µg m-2 min-1 (no added P) to 354 µg m-2 min-1 (high P). This pattern was not observed in autumn, as uptake rates of N were not measurable when P was elevated. Further, elevating background N concentration in either season did not significantly increase P uptake rates, likely because adsorption rather than biotic uptake dominated P dynamics. Laboratory P sorption assays demonstrated that Walker Branch sediments had a high adsorption capacity and were likely a sink for P during most pulse nutrient additions. Therefore, it may be difficult to use coupled pulse nutrient additions to evaluate biotic uptake of N and P in streams with strong P adsorption potential. Future efforts should use dual nutrient addition techniques to investigate reach-scale coupled biogeochemical cycles (C-N-P, and other elemental cycles [e.g., Fe, Mo]) across seasons, biomes, and land-use types and over longer time periods.
Contacts (BER PM)
Natalie A. Griffiths
Oak Ridge National Laboratory
This research was part of the long-term Walker Branch Watershed project and supported by the U.S. Department of Energy’s Office of Science, Biological and Environmental Research. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725. We thank T.V. Royer for partial laboratory support and we thank Indiana University’s School of Public and Environmental Affairs for supporting LTJ’s time.
Griffiths, N. A., and L.T Johnson. “Influence of dual nitrogen and phosphorus additions on nutrient uptake and saturation kinetics in a forested headwater stream.” Freshwater Science 37, 810-825 (2018). [DOI:10.1086/700700]
Data citation: Griffiths, N. A., and L.T. Johnson. 2018. Walker Branch Watershed: Effect of Dual Nitrogen and Phosphorus Additions on Nutrient Uptake and Saturation Kinetics, 2011-2012. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, U.S.A. [DOI:10.25581/ornlsfa.015/1484490]
Data are available on the ORNL TES SFA website: https://tes-sfa.ornl.gov/node/80#WBW_new
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